Nanotechnologies promise to bring about the advent of very small, yet important electronic and biological devices with features that are only a few tens of nanometers across. A variety of nanometer-scale (“nano”) materials, such as carbon nanotubes, nanoparticles, and molecular memories are being developed. However, improvements in the handling and patterning of these nanomaterials are necessary before they can be cost-effectively incorporated into useful nanodevices such as, for example, single-electron transistors, high-density gene chips, and terabyte-scale memory systems. These devices requite new fabrication and patterning techniques that far exceed resolution limitations of known processing techniques.
For example, known lithographic methods that are at the heart of modern microfabrication, nanotechnology, and molecular electronics often rely on patterning a resistive film, followed by chemical etching of the substrate. A variety of such subtractive printing techniques employ scanning probe instruments, electron beams, or molecular beams to pattern substrates using self-assembling monolayers and other organic materials to form sacrificial resistive layers. Known microfabrication techniques such as photolithography, microcontact printing, micromachining, and microwriting can produce patterns as small as 100 nm, but the production of sub-100 nm structures still poses a challenge.
Also, many nanomaterials containing discrete components, e.g. nanotubes, must ordinarily be manipulated and directly assembled onto a surface without the assistance of resists, masks, or etching steps. Furthermore, many organic materials that could be useful in nanoscale devices, such as DNA and proteins, are easily damaged and, thus, are difficult to pattern to form very small structures. Thus, new methods are needed to address the challenge of patterning and constructing useful nanoscale devices.
Since its inception, scanning probe microscopy has proven to be a useful tool for high-resolution imaging of nanoscale structures. The scanning probe typically includes a cantilever made of silicon having a length of about 200 μm. The cantilever has a sharp tip at its end with a radius of curvature generally below 10 nanometers. Depending on the imaging mode used, topological features as fine as individual atoms can be resolved.
More recently, it has been shown that the tip of a scanning probe microscope, such as an atomic force microscope (“AFM”), may be useful for the direct assembly of nanostructures. The tip can be very sharp, with only a few atoms located at its apex. A number of techniques that use an AFM tip to push very small objects, including atoms, nanoparticles, and nanotubes across a surface to form simple patterns, have been developed. However, the pushing operations are very complex, and construction of useful structures is indirect and often prohibitively tedious.
Another process, known as “dip pen nanolithography” (“DPN”), uses an AFM tip to deposit a restricted set of organic molecules onto carefully chosen substrates. Generally, DPN is a nanolithography technique by which molecules are directly transported to a substrate. DPN utilizes a solid substrate as the “paper” and an AFM tip (or a near-field scanning optical microscope tip) as the “pen.” The tip is coated with a patterning compound (the “ink”), and the coated tip is used to apply the patterning compound to the substrate to produce a desired pattern. The DPN delivery mechanism involves the formation of an adsorbed water meniscus around the tip to transfer the ink molecules to the substrate, and the control of the movement of the patterning molecules to the surfaces on which they are deposited by a driving force to form self-assembling monolayers.
Problems that arise with DPN technology stem from the dependence of this technique on the liquid meniscus and chemical affinity of the patterning material to the substrate. For example, the lateral width of the line written by the “pen” using DPN is limited by the width of the meniscus formed. The meniscus is subject to variations in the relative humidity as well as chemical interactions between the solvent and the substrate. The size of the meniscus may also affect the rate of the transport of the patterning compound to the substrate. Solubility characteristics of the “ink” molecules in a given solvent can create difficulty in establishing a desired line width and a suitable loading concentration of the ink in the solvent. Furthermore, surface tension characteristics of different solvents can lead to drip or rapid flow from the pen resulting in problems with precise control of the ink application under some circumstances. Finally, special substrate-liquid interactions and self-assembling chemistries may be necessary to promote the adhesion of the molecules to the substrate, limiting the kinds of materials that can be patterned in this fashion.
Thus, there remains an unresolved need in the art to enable rapid and direct patterning of arbitrary materials onto arbitrary substrates with nanoscale resolutions.